Carbene-stabilized 6,12-diboraanthanthrenes: unveiling the multistage redox properties of polycyclic aromatic hydrocarbons featuring electron-rich boron centers.

Electronic manipulation of boron centers in polycyclic aromatic hydrocarbon (PAHs) frameworks often leads to unique redox and photophysical properties. Herein, we report the first isolation and redox investigation of carbene-stabilized 6,12-diboraanthanthrenes 3/4 with electron-rich boron centers. Combining experimental and theoretical studies confirms that 3/4 exhibits a closed-shell singlet ground state and strong global aromaticity. Oxidation of 3/4 with TEMPO led to the oxidized product 5. Furthermore, the reversible one or two-electron oxidation process for 3/4 has been confirmed by cyclic voltammetry. Sequential oxidation of 3/4 with AgSbF results in the isolable radical monocations 6/7, and dications 8/9. The radical character of 6/7 is confirmed by multi-line EPR spectra, with the hyperfine coupling splitting mainly attributed to the two boron nuclei. As rare diboron-doped dicationic PAHs, 8/9 display bright yellowish fluorescence. Reduction of 3/4 with 2 equivalents of KC leads to the formation of the dianionic species 10/11, where the carbene ligands are initially reduced and subsequently reconstructed, confirming the electron-rich nature of boron centers.